1. Field of the Invention
The present invention relates to a micro oscillating element comprising an oscillation section capable of rotary displacement. The micro oscillating element of the present invention is applicable for producing a micromirror, an acceleration sensor, an angular speed sensor, and a vibrator, for example.
2. Description of the Related Art
In recent years, elements having a very fine structure formed by micromachining technology have found application in a variety of technological field. For example, very small micromirror elements having light reflection function have attracted attention in the field of optical communications technology.
In optical communications, optical signals are transmitted via optical fibers as a medium, and optical switching devices are generally used for switching the transmission path of optical signals from one fiber to another. A large capacity, high speed, and high reliability in switching operation are the characteristics required for the optical switching devices to realize good optical communication. From those standpoints, high hopes are pinned on the assemblies comprising micromirror elements as the optical switching devices fabricated by micromachining technology. This is because the micromirror elements can conduct switching of optical signals directly, that is, without converting the optical signals into electric signals, between the input optical transmission path and output optical transmission path in the optical switching device and are advantageous in terms of obtaining the above-described characteristics.
A micromirror element can comprise a mirror surface for reflecting the light, and the reflection direction of the light can be changed by the oscillation of the mirror surface. Micromirror elements of an electrostatic drive type that use an electrostatic pull-in force for turning the mirror surface have been used in a large number of devices. The micromirror elements of an electrostatic drive type can be generally classified into two groups: micromirror elements fabricated by the so-called surface micromachining technology and micromirror elements fabricated by the so-called bulk micromachining technology.
With the surface micromachining technology, various parts constituting the element, such as a supporting and fixing sections, oscillation section, mirror surface, and electrode sections, or a sacrificial layer that is subsequently removed are formed in a substrate by machining thin material films corresponding to each structural part to the desired pattern and then successively laminating the patterns. On the other hand, with the bulk micromachining technology, the fixing and supporting section or oscillation section are formed to the described shape by etching the material substrate itself and then the mirror surface or electrodes are formed as thin films. The bulk micromachining technology is described, for example, in Japanese Patent Applications Laid-open Nos. H09-146032, H09-146034, H10-190007, and 2000-31502.
A high flatness of the mirror surface serving to reflect the light is one of the technological characteristics required from the micromirror element. However, with the surface micromachining technology, the mirror surface that is finally formed is thin and the mirror surface can be easily bent. Therefore, a high degree of flatness is difficult to attain on the mirror surface of a large area. By contrast, with the bulk micromachining technology, a material substrate of a comparatively large thickness is cut by etching to form a mirror supporting section and a mirror surface is provided on the mirror supporting section. Therefore, even if the mirror surface has a large area, the rigidity thereof can be ensured. As a result, a mirror surface of a sufficiently high degree of optical flatness can be formed.
FIGS. 40-43 illustrate an example of a micromirror element (generally indicated by X4), as related art, which can be manufactured by a conventional method. FIG. 40 is a plan view of the micromirror element X4. FIGS. 41-43 are the cross-sectional views along the lines XXXXI-XXXXI, XXXXII-XXXXII, and XXXXIII-XXXXIII in FIG. 40, respectively.
The micromirror element X4 comprises an oscillation section 80, a frame 91, a pair of oscillating bars 92, and a comb-tooth electrode 93 and is manufactured by conducting the below-described machining of a material substrate that is the so-called SOI (silicon on insulator) substrate by a bulk micromachining technology. As described below, the material substrate has a laminated structure composed of silicon layers 201, 202 and an insulating layer 203 located therebetween. The silicon layers 201, 202 are provided with the prescribed electric conductivity by doping with dopants. The aforementioned various parts of the micromirror element X4 are formed from the silicon layer 201 and/or silicon layer 202. In order to make the figure clearer, in FIG. 40, the parts derived from the silicon layer 201 and protruding forward from the paper sheet with respect to the insulating layer 203 are provided with hatching.
The oscillation section 80, for example, as shown in FIG. 43, is a part derived from the silicon layer 201. It has a mirror support section 81, a comb-tooth electrode 82, and a beam section 83. A mirror surface 81a having a light reflection function is provided on the surface of the mirror support section 81. The comb-tooth electrode 82 is composed of a base section 82a and a plurality of electrode teeth 82b extending from the base section. The beam section 83 joins the mirror support section 81 and comb-tooth electrodes 82 and is electrically connected to these two sections.
The frame 91 is a part mainly derived from the silicon layers 201, 202, as shown in FIGS. 41-43. It has a shape surrounding the oscillation section 80 and supports the structure located inside the frame 91.
A pair of oscillating bars 92 are the parts derived from the silicon layer 201. They are connected to the beam section 83 of the oscillation section 80 and the parts derived from the silicon layer 201 in the frame 91 and join them. Each oscillating bar 92 electrically connects the beam section 92 and the parts derived from the silicon layer 201 in the frame 91. Such a pair of oscillating bars 92 defines an oscillation axis A4 for the oscillating action of the oscillation section 80 through mirror support section 81.
The comb-tooth electrode 93 is a part for generating an electrostatic pull-in force in cooperation with the comb-tooth electrode 82 and is composed of a plurality of electrode teeth 93a extending from the frame 91. The electrode teeth 93a are the parts derived from the silicon layer 202 and are fixed to the parts derived from the silicon layer 202 in the frame 91. Such comb-tooth electrode 93 and the above-described comb-tooth electrode 82 constitute a drive mechanism of the present element. For example, in a non-operative state of the oscillation section 80, the comb-tooth electrodes 82, 93 are positioned at different heights, as shown in FIG. 42 and FIG. 43. Furthermore, the electrode teeth 82b, 93a are arranged with a shift with respect to each other so that the comb-tooth electrodes 82, 93 are not in contact with each other during the oscillating action of the oscillation section 80.
In the micromirror element X4, the oscillation section 80 through mirror support section 81 can be caused to rotate around the oscillation axis A4, if necessary, by applying the prescribed electric potential to each comb-tooth electrode 82, 93. The application of electric potential to the comb-tooth electrode 82 can be realized via the parts derived from the first silicon layer of the frame 91, both oscillating bars 92, and beam section 83. The application of electric potential to the comb-tooth electrode 93 can be realized via the parts derived from the second silicon layer of the frame 91. If a desired electrostatic pull-in force is generated between the comb-tooth electrodes 82, 93 by applying the prescribed potential to the comb-tooth electrodes 82, 93, the comb-tooth electrode 82 is pulled in to the comb-tooth electrode 93. As a result, the oscillation section 80 through mirror support section 81 rotate around the oscillation axis A4 and the rotary displacement is induced till the angle is attained that provides for the balance between the electrostatic pull-in force between the comb-tooth electrodes 82, 93 and the sum of the twisting resistance forces of the oscillating bars 92. Further, if the electrostatic pull-in force acting between the comb-tooth electrodes 82, 93 is canceled, the oscillating bars 92 return to the natural state and the oscillation section 80 through the mirror support section 81 assume the orientation shown in FIG. 43. The above-described oscillating drive of the oscillation section 80 through mirror support section 81 makes it possible to switch appropriately the reflection direction of light reflected by the mirror surface 81a provided on the mirror support section 81.
FIG. 44 shows part of the process for the manufacture of the micromirror element X4. In FIG. 44, the process of forming part of the mirror support section 81, frame 91, oscillating bar 92, and part of the set of comb-tooth electrodes 82, 93 shown in FIG. 40 is represented as changes in one cross-section. This one cross-section is represented as a continuous cross-section obtained by simulating the cross-sections in a plurality of prescribed locations contained in a single micromirror element formation area in the material substrate (wafer having a multilayer structure) that is to be machined.
In the manufacture of the micromirror element X4, first, a material substrate 200 shown in FIG. 44A is prepared. The material substrate 200 is a SOI wafer and has a laminated structure composed of silicon layers 201, 202 and an insulating layer 203 located therebetween. Then, as shown in FIG. 44B, the mirror support section 81, parts of frame 91, oscillating bar 92, and comb-tooth electrode 82 are formed in the silicon layer 201 by conducting anisotropic etching of the silicon layer 201 via the prescribed mask. Then, as shown in FIG. 44C, part of the frame 91 and the comb-tooth electrode 93 are formed in the silicon layer 202 by conducting anisotropic etching of the silicon layer 202 via the prescribed mask. Then, as shown in FIG. 44D, zones exposed in the insulating layer 203 are removed by conducting anisotropic etching of the insulating layer 203. The oscillation section 80 (mirror support section 81, comb-tooth electrode 82, beam section 83), frame 91, oscillating bar 92, and comb-tooth electrode 93 are thus formed.
As described above, the oscillation section 80 is a part derived from the silicon layer 201, and the frame 91 has a part derived from the silicon layer 201 and a part derived from the silicon layer 202. For this reason, in the micromirror element X4, a gap has to be provided between the oscillation section 80, which is a movable section, and the frame 91, which is the fixed section, and those components have to be separated in the in-plane direction of the material substrate. The length of this gap between the oscillation section 80 and frame 91 has to be set above the prescribed level. For example, the length d4 between the oscillation section 80 and frame 91 of the gap G provided between the mirror support section 81 of the oscillation section 80 and the frame 91 has to be set above the prescribed level so that the material between the mirror support section 81 and frame 91 in the silicon layer 201 can be adequately etched out in the process described hereinabove with reference to FIG. 44B.
The smaller is the distance d4, the larger is the aspect ratio D/d4 (D is the thickness of the silicon layer 201) of the gap G that has to be formed between the mirror support section 81 and frame 91 in the process described hereinabove with reference to FIG. 44D. When the length d4 is less than the prescribed level and the aspect ratio D/d4 is larger than the prescribed level, the material between the mirror support section 81 and frame 91 is difficult to etch out adequately. As a result, the mirror support section 81, part of the frame 91, and gap G located therebetween are difficult to form adequately. Therefore, the length d4 of the gap G between the mirror support section 81 and frame 91 has to be increased to a degree ensuring a sufficiently small aspect ratio.
In such a micromirror element X4 in which the length d4 of the gap G between the mirror support section 81 and frame 91 has to be set above the prescribed level, miniaturization by reducing the size in the direction of the oscillation axis A4 and direction perpendicular thereto is sometimes difficult to attain.